1. Field of the Invention
This invention relates to the clamping of a signal to a reference level and in particular to the periodic clamping of a video signal to a reference dc level.
2. Description Relative to the Prior Art
It is necessary, after ac coupling, to restore the dc level of a video signal at selected points in a video circuit. In a video system employing frequency-modulation, for instance, it is important that the dc level be correct prior to the clipping and modulating stages. Otherwise the clipper may attenuate more of the signal than it should and the frequency modulator may produce an output with respect to the wrong frequencies.
Maintenance of correct dc level is of additional significance in the case of a color signal generated from line sequential color difference signals, that is, R-Y (red-luminance) and B-Y (blue-luminance) signals in alternating line sequence: It is necessary to provide some manner of identifying each color difference signal and of maintaining that identification throughout the generation, transmission (or recording) and reception (or playback) of such signals. The identification may be provided by an index signal which is characteristic of the onset of one of the color difference signals (see, for example, the system outlined in U.S. Pat. No. 4,470,076). If the color difference signals modulate the frequency of a carrier, such identification may consist of different rest frequencies for the R-Y and B-Y color difference signals. Since a typical frequency modulator converts a voltage to a frequency, the different rest frequencies may correspond to different dc blanking level voltages for the R-Y and B-Y signals.
A line sequential color signal, moreover, is sensitive to errors in its blanking level because its component signals are significantly different in each new line. For that reason U.S. Pat. No. 4,114,179 suggests a clamping circuit that clamps on each individual line, thus assuring that the dc level is correct for each line. In a system where the blanking level is also the signal identifier, there is further reason to clamp each signal to respective blanking levels prior to clipping and modulating. With separate blanking levels, however, more complex clamping circuitry is required. It is burdensome, furthermore, to clamp a train of signals to alternately varying dc levels since the short existence of each clamping interval (approximately 10.2 microseconds is available within the horizontal blanking period) requires a very fast reacting clamp with virtually no allowance for settling time. In other words, the clamp has but one chance to be on the mark or the system fails.